Solar Cycle Occurrence of Alfvénic Fluctuations and Related Geo-Efficiency

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Details

Original languageEnglish
Pages (from-to)9848-9857
Number of pages10
JournalJournal of geophysical research: Space physics
Volume122
Issue number10
Early online date17 Oct 2017
Publication statusPublished - 2017
MoE publication typeA1 Journal article-refereed

Researchers

Research units

  • Finnish Meteorological Institute
  • Space Science Institute, Boulder, CO, USA
  • University of Bergen
  • University of Michigan, Ann Arbor
  • University College of London
  • Max‐Planck‐Institute for Solar System Research
  • University of Oulu

Abstract

We examine solar wind intervals with Alfvénic fluctuations (ALFs) in 1995–2011. The annual number, the total annual duration, and the average length of ALFs vary over the solar cycle, having a maximum in 2003 and a minimum in 2009. ALFs are most frequent in the declining phase of solar cycle, when the number of high-speed streams at the Earth's vicinity is increased. There is a rapid transition after the maximum of solar cycle 23 from ALFs being mainly embedded in slow solar wind (<400 km/s) until 2002 to ALFs being dominantly in fast solar wind (>600 km/s) since 2003. Cross helicity increased by 30% from 2002 to 2003 and maximized typically 4–6 h before solar wind speed maximum. Cross helicity remained elevated for several days for highly Alfvénic non-ICME streams, but only for a few hours for ICMEs. The number of substorms increased by about 40% from 2002 to 2003, and the annual number of substorms closely follows the annual cross helicity. This further emphasizes the role of Alfvénic fluctuations in modulating substorm activity. The predictability of substorm frequency and size would be greatly improved by monitoring solar wind Alfvénic fluctuations in addition to the mean values of the important solar wind parameters.

    Research areas

  • Solar wind plasma, Plasma waves and turbulence, Solar cycle variations, Substorms, Magnetic fields and magnetism, Alfvénic fluctuations, Space climate, Geomagnetic activity, Solar cycle time scales, High-speed streams

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